1. Field of the Invention:
This invention relates to a heat-insulating structure of a swirl chamber in an internal combustion chamber.
2. Description of the Prior Art:
Generally, in combustion chambers of a swirl chamber type in an internal combustion engine, mixing of a fuel and air is made twice each in a swirl chamber and main combustion chamber and the mixing state is better than in those of a direct injection type. However, the loss of cooling water is greater with the swirl chamber type than with the direct injection type and the fuel efficiency becomes lower. Therefore, attempts have been made to constitute the swirl chamber in a heat-insulating structure in order to minimize the loss of cooling water.
However, in the case of the heat-insulating structure wherein the outer surface of the swirl chamber is heat-insulated uniformly, cracks, breakage, etc., of the swirl chamber occur due to thermal stress resulting from temperature difference that occurs in the swirl chamber. Thus, the problem of durability of the swirl chamber arises. The temperature distribution on the inner wall surface constituting the swirl chamber is such that the temperature becomes high particularly at jet port formation portions of jet ports communicating from the main combustion chamber to the swirl chamber, and the temperature distribution around the jet port portion provides high temperature particularly at the jet port portion on the center side of the main combustion chamber. Accordingly, the temperature distribution around the jet port formation portions of the swirl chamber block is different to some extents when the swirl chamber block for constituting the inner wall portion of the swirl chamber is made of a ceramic material and a thermal stress occurs. This thermal stress exerts adverse influences on the strength of the ceramic material and poses a durability problem. For this reason, a problem remains to be solved as to how the swirl chamber itself be constituted in order to improve durability of the swirl chamber block.
A production method of a swirl chamber of an engine is known in the past from Japanese Patent Laid-Open No. 83451/1986, for example. The production method of the swirl chamber of the engine disclosed in this prior art reference fits an outer cylinder of an iron type sintered material which is subjected to compression powder molding or preparatory sintering on an inner cylinder made of ceramic and then couples the inner and outer cylinders integrally by regular sintering to produce the swirl chamber of the engine. Namely, an insert member is prepared by integrating ceramic particles by use of a copper type bonding material and molding the integrated member in a shape substantially equal to the shape of a heat-insulating chamber to be formed in a predetermined position between the inner and outer cylinders described above, and after this insert member is interposed to the predetermined position between the inner and outer cylinders, the regular sintering step is carried out.
In the production method of the swirl chamber of the engine described above, the outer peripheral metal material consists of the sintered material. Therefore, the sintered metal has the function of only sealing the heat-insulating layer but cannot control the compressive force or the heat-insulating degree. In other words, this production method does not have the technical concept of improving durability of the inner cylinder made of the ceramic material.
A swirl chamber structure of a Diesel engine is disclosed in Japanese Utility Model Laid-Open No. 87226/1988, for example. This swirl chamber structure of the Diesel engine constitutes the swirl chamber by burying a shell-like member and a hot plug into the cylinder head or the cylinder block. In this swirl chamber structure of the engine, the shell-like member and the hot plug are composed of a heat-resistant steel, and a porous ceramic layer is formed by spraying on the outer surfaces of both, or either one, of these members and they are buried into the cylinder head or the cylinder block so as to constitute the swirl chamber.
In the swirl chamber structure of the Diesel engine described above, the block constituting the swirl chamber is made of a metallic material and the ceramic layer is disposed on the outer peripheral portion of this block. In other words, the block itself is not constituted by the ceramic layer and this prior art technique lacks the technical concept of improving durability of this block.
When the block constituting the swirl chamber is directly casted into the large member such as the cylinder head as is made in the prior art technique, deviation of the dimension at the time of casting becomes excessively great and such a product cannot be employed as the product. The deviation of the cast dimension must be about .+-.1.5 mm for the size of about 500 mm but accuracy of position dimension of the swirl chamber must be about .+-.0.2 mm.
Residual compressive stress cannot be applied to the ceramic material by shrinkage fit of the metallic material to the ceramic material because the application direction of the compressive stress is unidirectional. Thus, the effective residual compressive stress cannot be applied to the ceramic material.